QUESTION: Rebbi Yochanan says that if one inadvertently has relations with
one's wife while she is a Nidah, he is Chayav to bring a Chatas. If one
inadvertently has relations with his Yevamah (the wife of his deceased
brother, who left behind no living children after his death) who is a Nidah,
he is Patur, because he is "To'eh b'Devar Mitzvah."
The Gemara gives two explanations why one is Chayav in the case of one's
wife who is a Nidah: (1) either because he did not accomplish any Mitzvah,
and one who is "To'eh b'Devar Mitzvah *v'Lo Asah Mitzvah*" is not Patur, or
(2) because his mistake in this case is not an acceptable mistake, because
he should have asked his wife if she was a Nidah. Therefore, his act is
considered a Shogeg and not a mistake ("To'eh") and he is Chayav a Chatas.
The Gemara then asks why is a person Chayav in this case -- he should be
Patur, because he is fulfilling the Mitzvah of having children (Piryah
v'Rivyah), or the Mitzvah of Onah, and therefore he should be considered a
"To'eh b'Devar Mitzvah" and he should be Patur.
What is the Gemara asking? The Gemara just explained that there are two ways
to understand Rabbi Yochanan's statement, and this question is not
appropriate according to either way. If a woman is a Nidah, one has not
fulfilled the Mitzvah of Piryah v'Rivyah, nor has he fulfilled the Mitzvah
of Onah. If so, why is it considered a case of "Asah Mitzvah?" He has *not*
accomplished any Mitzvah and therefore he is certainly Chayav!
According to the second way of understanding Rebbi Yochanan's statement, the
Gemara already explained that since he should have asked his wife if she was
a Nidah, his act cannot be considered a mistake. Why, then, is the Gemara
saying that he should be Patur because of "To'eh b'Devar Mitzvah"? (TOSFOS,
TOSFOS explains that the Gemara is actually asking this question on the
second way of understanding Rebbi Yochanan. According to that approach, even
"To'eh b'Devar Mitzvah v'Lo Asah Mitzvah" is Patur, and thus one who has
relations with his wife who is a Nidah, thinking that he is fulfilling a
Mitzvah and unaware that she is a Nidah, should be Patur even though he did
not actually accomplish any Mitzvah. To this the Gemara asks why indeed is
he Chayav if he intended to do a Mitzvah.
What about the fact that the Gemara already explained that he is *not*
considered a "To'eh b'Devar Mitzvah" because he should have asked her if she
was a Nidah? The TOSFOS SHANTZ explains that when the Gemara said that he
should have asked her, the Gemara already *knew* the conclusion that the
Gemara would express later -- that he should have asked her if she was
Samuch l'Vestah or a Nidah. The Gemara is just explaining how it came to
that conclusion, and in doing so, it goes through all of the other options
and explains why they are not viable (that is, it cannot be that she is
pregnant, because there is still a Mitzvah of Onah, and so on).
(It is not clear whether RASHI agrees to this reasoning. In (DH Ishto Nidah)
Rashi seems to explain that the Gemara's question is even according to the
*first* way of understanding Rebbi Yochanan. Rashi prefaces *both* of the
explanations for Rebbi Yochanan's statement by pointing out that the reason
why the case in question is not considered Ta'ah b'Devar Mitzvah is because
"she was pregnant and Samuch l'Vestah" (DH Ishto Nidah).")